The present invention relates to moisture sensors and, more particularly, to moisture sensors for sensing moisture on an automobile windshield.
The sensor operates on a differential capacitive principle with electrodes deposited on the internal surface of the windshield, which results in higher immunity to undesired influences, and selective response to moisture deposited on either surface of the windshield.
Prior art automotive windshield moisture sensors are invariably based on a single electro-optical concept typically described in U.S. Pat. No. 5,917,603 to Tanaka. In this method, a rain droplet is sensed by measuring the change in the total internal reflection of a light beam off the glass-air interface of the front surface. This method has a limited sensing area, is relatively expensive, and is only suitable for sensing moisture on the outside surface of the windshield.
U.S. Pat. No. 5,923,027 to Stam et al., describes an electro-optical method wherein the brightness of a section of the window image is analyzed to detect rain droplets or fog on the window. A similar approach is disclosed in U.S. Pat. No. 5,020,704 to Buschur.
Attempts have been made to detect rain based on its effect on the resistance, or capacitance, between electrodes deposited on the windshield. Sensors based on these methods are integral with the windshield and are potentially less expensive and less conspicuous.
In U.S. Pat. No. 5.739.430 to Berberich, resistance sensing electrodes are deposited on the external surface of the windshield and are subjected to wear. Extra protection is provided by a sintered glass layer as taught in U.S. Pat. No. 5.783,743 to Weber.
The capacitance sensing method relies on the relatively large dielectric constant of water (approximately 80) as it affects the capacitance between conductive electrodes. The method is described in U.S. Pat. No. 4,805,070 to Koontz et al., U.S. Pat. No. 4,831,493 to Wilson et al., and U.S. Pat. No. 5,668,478 to Buschur.
Typical moisture sensors in which the conductive electrodes are deposited inside the laminate of the xe2x80x9csandwichedxe2x80x9d windshield glass are described in U.S. Pat. No. 4,703,237 to Hochstein, in U.S. Pat. No. 4,827,198 to Mueller. in U.S. Pat. No. 4,613,802 to Krause et al., and in U.S. Pat. No. 4,554,493 to Armstrong. In these devices, the capacitive effect of water drops changes the resonant frequency of a resonant circuit.
In prior art capacitive moisture sensors, in which the full thickness of the glass separates the capacitor plates from the water-droplets, the relative change of the capacitance due to water drops is very small, and until recently these devices had difficulty of distinguishing between real signal due to water and false signal due to fluctuations in the window temperature which affects its dielectric constant.
The capacitive rain sensor of U.S. Pat. No. 3,826,979 to Steinmann, uses electrodes deposited on the internal surface of the window in order to sense rain on the external surface of the windshield. In Steinmann""s invention, three electrodes are used to shape the fringing electric field in order to increase the signal due to raindrop relative to signal due to the glass. The improvement is marginal since the majority of the measured capacitance is still due to the glass whose temperature dependence still swamps any signal due to rain drops.
In summary, depositing the capacitive plate on the outside surface is not reliable because of the wiper abrasion. deposition in the interlayer is difficult to terminate electrically, and depositing on the inside surface is protected and reliable, but the signal is small. A further disadvantage of prior-art capacitive moisture sensors. regardless of the position of the plates, was their inability to distinguish between moisture on the internal and on the external surfaces of the windshield, making it impossible to automate both the windshield wiper and the defogging means.
For these reasons, capacitive moisture sensors were not in use in the automotive applications. It was not until recently that most of the drawbacks of the capacitive sensors were overcome, as described in U.S. Pat. Nos. 5,801,307 and 5,682,788 and in EP application; EP 0753438, all to Netzer. The improved sensors are stable with temperature, do not obstruct the driver""s view, and are xe2x80x9cdirectionalxe2x80x9d, i.e., they can be made selectively sensitive to moisture on either surface of the windshield. Yet, these capacitive sensors have the drawback in that the capacitor plates are embedded within the laminated layer of the front windshield. Such structure imposes limitations on their manufacture and reduce their usage, because not all car windows are laminated. Besides, it is difficult to establish electrical connections to the embedded electrodes.
There is, therefore, a need for a capacitive rain sensor that would overcome the disadvantages of presently known sensors, as described above.
According to the present invention there is provided an improved automotive raindrop sensor integrally manufactured with the window, with an optional capability to sense moisture on the internal surface of the window. The sensor of the present invention, is inexpensive, does not obstruct the drivers view, is not subject to abrasion, is insensitive to temperature effects on the glass, and is also substantially insensitive to nearby conductive or dielectric objects.
The moisture sensor of this invention is capable of sensing moisture over a large surface area, unlike conventional electro-optical moisture detectors which sample moisture only in a small region, additionally it does not generate false wipe command when touched with a finger.
There is provided a temperature stable capacitive sensor having a sensitive area, for sensing moisture on the external surface of a window having a certain temperature. the sensor comprising: (a) a conductive pattern including at least three capacitive plates disposed on the internal surface of the window, the at least three plates defining at least one sensing capacitor and at least one compensation capacitor; (b) means for generating an output level which changes in response to the moisture and is proportional to the difference between the at least one sensing capacitor and the at least one compensation capacitor; and (c) a shield for protecting at least the pattern from direct moisture condensation and from electrical interference.
There is provided a temperature stable capacitive sensor having a sensitive area for sensing moisture on the internal surface of a window having a certain temperature comprising: (a) a conductive pattern including at least three capacitive plates disposed on the internal surface of the window, the at least three plates defining at least one sensing capacitor and at least one compensation capacitor; (b) means for generating an output level which is indicative of the moisture and is proportional to the difference between the at least one sensing capacitor and at least one compensation capacitor; and (c) a shield for protecting at least the compensation capacitor from direct moisture condensation and from electrical interference.
Other objects of the invention will become apparent upon reading the following description taken in conjunction with the accompanying drawings.